The present invention relates to thin-surfaced assemblies, the thin surfaces or skins of which are subjected to severe temperature levels and gradients during use, which cause expansion, distortion and/or erosion of the heated surface areas, and to internal support structures for such assemblies. Most particularly, the present invention relates to improved gas turbine nozzle vanes or blades.
As disclosed in U.S. Pat. No. 3,215,511, nozzle vanes or blades are subjected to a continuous contact of effluent through convective and/or impingement air directed against their leading edges and/or other surfaces, which tends to erode these edges and produces severe localized temperature gradients and/or high temperatures in the vanes or blades at the leading edges and also in contiguous sections of the trailing portions. This causes the surface of the vanes or blades to expand unevenly and creates stresses which can distort or warp the blades, reducing their aerodynamic performance and leading to cracking or permanent warpage and substantial reduction in the useful life of such airfoils.
Some of these problems are reduced, such as by the use of cast cores having ceramic inserts to reduce erosion as in said U.S. Pat. No. 3,215,511, and/or by providing the blades with a hollow case core having a longitudinal aperture for the passage of coolant through the blade, as disclosed in U.S. Pat. No. 4,247,259. The most common system, in use today is the provision of the turbine vanes or blades with internal cooling air passages which communicate with the interior surface and portions of the outer skin to provide impingement and/or film cooling as in U.S. Pat. No. 4,022,542, and/or which communicate with air bleed passages which extend through the outer skin for greater cooling efficiency as in U.S. Pat. Nos. 3,732,031 and 4,118,146, referred to as transpiration cooling. Blades which do not have cast metal cores, as in said Patent 4,022,542, do not have sufficient strength for practical use since their strength depends upon the strength of the facing or skin materials. Known skin materials which are not supported by a flexible core will crack after a relatively short period under the conditions of use.
It is the objective of the present invention to provide a novel design for structures having thin skin surfaces which are subjected to severe temperature levels and gradients during use, which design avoids or compensates for distortional stresses which normally accompany the uneven severe temperature gradients and yet provides sufficient strength and internal support and flexibility for the skin surfaces to prevent cracking thereof during use, i.e., a design which is less concerned with increasing the cooling of the structure than with reducing the adverse stress effects of gradient or uneven heating thereof.